This disclosure is related to the field of drilling wellbores through subsurface formations. More specifically, the disclosure relates to methods and systems for simultaneous drilling of a wellbore while inserting a protecting pipe or casing into the drilled wellbore.
Wellbore drilling through subsurface formations known in the art includes so-called “casing drilling” or “casing while drilling” systems and methods. Such systems and methods enable simultaneous drilling of a wellbore through the formations and insertion into the drilled wellbore of a protective pipe or casing. The casing may be cemented in place after the wellbore is drilled to its intended depth, and serves, among other functions, to protect the mechanical integrity of the wellbore and to provide hydraulic isolation between formations traversed by the wellbore.
Casing while drilling systems known in the art are described, for example, in U.S. Pat. No. 8,534,379 issued to Giroux et al., U.S. Pat. No. 7,624,820 issued to Angman et al. and U.S. Pat. No. 7,475,742 issued to Angman et al. In casing drilling methods and systems known in the art, the casing may be used to transport drilling fluid (“mud”) from the surface to a drill bit disposed at an end of a bottom hole assembly (BHA) consisting of various drilling and hole diameter enlarging (underreaming) tools. As the drill bit lengthens the wellbore, and the underreamer enlarges the wellbore diameter to enable movement of the casing therethrough, drill cuttings are lifted and transported by the drilling mud from the bottom of the wellbore and the position of the underreamer to the surface through an annular space (“annulus”) between the casing and the wellbore. Some casing drilling systems may omit the use of an underreamer. See, e.g., the Giroux et al. '379 patent referred to above.
As is well known in the art, it may be undesirable to have a large annulus in order to provide good conditions for later cementing of the casing in the wellbore. Having a relatively small annulus, however, makes transport of the cuttings to the surface more difficult and may even increase the risk that the casing becomes stuck in the wellbore before reaching the intended well depth.